Originally, semiconductor lasers were diode structures in which light emitted from the edge of the laser structure was parallel to the surface of the semiconductor wafer. However, this edge-emitting laser structure does not lend itself to cost-effective fabrication of two-dimensional arrays of laser diodes. A second class of laser diodes, well suited for fabrication of laser arrays, is fabricated such that the laser structure is perpendicular to the surface of the semiconductor wafer so that light is emitted perpendicular to the surface. These lasers in this class of lasers are commonly known as surface emitting lasers (SELs).
Both classes of lasers are formed on a substrate which may be semi-insulating or may be conductive with either p-type or n-type doping. FIG. 1A shows a cross-sectional view of the conventional n-drive SEL 100 formed on the semi-insulating substrate 102. The surface emitting laser 100 may be regarded as an n-i-p diode, comprised of an n-type mirror region 104, an active region 106, and a p-type mirror region 108. Electrical connections are made via the electrode 110 formed on the top surface of the n-type mirror region 104 and electrode 112 formed on the p-type mirror region 108.
To make electrical contact with the p-type region 108, an etch is made through both the n-type mirror region 104 and the active region 106 to the p-type region 108. This is problematic since the p-type contact etch exposes the epitaxial layers 104, 106 and 108 which, when exposed, have a tendency to oxidize. Further, the p-type contact etch creates a non-planar structure which leads to potential reliability problems and increased manufacturing complexity. Further, defects added to semi-insulating substrates to make the substrate isolating can reduce the reliability of the semiconductor laser device.
FIG. 1B shows a cross-sectional view of the conventional n-drive surface emitting laser 120 formed on the p-type substrate 122. The SEL is comprised of the n-type mirror region 124, the active region 126, and the p-type mirror region 128. Electrical connections are made via the electrode 130 formed on the surface of the n-type mirror region 124 and the electrode 132 formed on the surface of the p-type substrate 122, remote from the p-type mirror region 128.
The preferred method of forming the n-type, i, and p-type regions is by molecular beam epitaxy (MBE). The only commonly-available p-type group III-V substrate is doped with zinc. However, at typical MBE growth temperatures, zinc diffuses out from the substrate, which causes an unacceptable background concentration in the mirror regions 124 and 128 and the active region 126. Further, zinc diffusing out from the substrate contaminates the molecular beam epitaxy chamber, which requires the addition of a cleaning step after each zinc contamination. Finally, p-type GaAs substrates are 3-4 times more expensive than n-type GaAs substrates, have a greater etch pit density, and are less readily available.
FIG. 1C shows a cross-sectional view of the surface emitting laser 140 formed on the n-type substrate 142. The SEL is comprised of the n-type mirror region 144, the active region 146, and the p-type mirror region 148. The SEL 140 shown in FIG. 1C is a p-drive SEL. Unlike the n-drive current driven SELs shown in FIGS. 1A and 1B, the p-drive SEL is typically voltage driven. Although current drivers for p-drive SELs exist, they have problems. For example, available silicon pnp drivers typically have insufficient speed to operate at the data rates of current optical communication systems. GaAs pnp drivers are faster but expensive.
Voltage driven p-drive SELs also have problems. Voltage driven p-drive SELs in SEL arrays require precise manufacturing control to make each laser in the array have the same V.sub..function.. Nonuniformities in V.sub..function. require that each laser in the array be individually pre-biased, which increases the cost of the laser drivers.
N-drive SELs may be created from the structure shown in FIG. 1C by sawing between individual lasers and flipping the p-drive SELs. However, this method cannot be used to make SEL arrays.
High-intensity light-emitting diodes (LEDs) can be used in many applications instead of lasers. Light-emitting diodes are simpler to manufacture than lasers and are consequently lower in cost. Light-emitting diodes may be made by replacing the n-type mirror region and the p-type mirror region in the structures just described with a homogeneous layer of n-type semiconductor material and a homogeneous layer of p-type semiconductor material, respectively. N-drive LEDs have advantages over p-drive LEDs similar to the advantages of n-drive lasers over p-drive lasers discussed above.
Near infra-red LEDs operating in the 800-880 nm range typically consist of a GaAs/AlGaAs double heterojunction structures grown epitaxially lattice-matched on GaAs substrates. Since GaAs is opaque to the light generated by the LED, the LEDs are typically formed to emit light from their top surface to save having to etch a hole through the substrate. Light emission from the top surface of the LED requires either a transparent top electrode or an annular top electrode to funnel the current into a region defined by a buried confinement or current-blocking layer. Irrespective of the configuration of the top electrode, the structures just described have additional series resistance compared with the direct vertical current injection geometry of back-surface emitting LEDs. The additional series resistance is a significant problem in LEDs because of the higher operating currents of typical LEDs compared with typical lasers. The additional series resistance increases the voltage required to drive the LEDs, increases the heat dissipated in the LED, and reduces the overall electro-optical efficiency of the LED.
One known approach to reducing the series resistance of top-emitting LEDs using an annular top electrode is to form the LED by growing an n-type AlGaAs layer using organo-metallic vapor-phase epitaxy (OMVPE) on a p-type GaAs substrate. This structure has less series resistance because the sheet resistivity that can be achieved in n-type AlGaAs is substantially lower than that which can be achieved in p-type AlGaAs. The sheet resistivity of n-type AlGaAs is lower than that of p-type AlGaAs primarily because the mobility of electrons is substantially greater than that of holes. However, the structure just described provides the advantage of lower resistance at the expense of requiring a p-type substrate, which has the disadvantages discussed above.
Accordingly, a way to form n-drive semiconductor light-emitting devices such as surface-emitting lasers and LEDs on an n-type substrate is needed so that the advantages of faster drive speed and lower series resistance of n-drive devices can be obtained without the disadvantages of a p-type substrate.